The present invention relates to a Raman excitation control method and optical transmission system using the same.
In FIG. 1, there is shown a configuration example of optical transmission system using optical amplification by means of Raman excitation control. In the system shown in this figure, an optical amplification system 1 is connected to an oppositely located optical amplification system 2 through a bi-directional optical transmission line 3.
For example, in FIG. 1, an optical signal is fed into an optical transmission line 3 through an optical amplifier 10 constituted by Erbium-doped fiber in optical amplification system 2, reaching an optical amplifier 20 in optical amplification system 2. Here, optical amplifier 20 is also constituted by Erbium-doped fiber.
On the contrary, an optical signal from optical amplification system 2 is fed into optical transmission line 3 through an optical amplifier 21, reaching an optical amplifier 11 in optical amplification system 1.
Bi-directional optical transmission line 3 connecting optical amplification system 1 with optical amplification system 2 has a transmission distance of approximately 200 km. For this purpose, optical amplification technology using Raman excitation light source has been introduced so that the optical signal is transmitted with a sufficient gain from optical amplification system 1 to optical amplification system 2, and oppositely from optical amplification system 2 to optical amplification system 1.
This optical amplification using Raman excitation light source is a technology in which the optical transmission line is utilized as an optical amplifier, which is similar to the Erbium-doped fiber amplification used for optical amplifiers 10, 21, 20 and 11. For this purpose, as shown in FIG. 1, there are provided Raman excitation light sources 12, 22 so as to perform backward excitation against optical signals for transmission.
In FIG. 2, there is shown a relation between the wavelengths of Raman excitation light and the gains. The gain produced by Raman amplification gradually increases from Raman excitation light wavelength I. In the case of 1550 nm band, the gain characteristic becomes maximum at the wavelength approximately 110 nm longer than the wavelength I. Accordingly, the Raman excitation light wavelength I is determined so that the optical wavelength of a main signal II is allocated in the area in which the maximum gain is produced.
In recent years, a wavelength-multiplexing optical transmission system using a plurality of main signals has been introduced. In such a system, a plurality of main signals are allocated in a main signal wavelength area. A plurality of Raman excitation light wavelengths are provided corresponding to the main signals.
In FIG. 3, there is shown an amplification gain characteristic produced by a plurality of Raman excitation lights. Corresponding to a plurality of main signal wavelengths II-1 to II-4, a plurality of laser diodes LD1 to LD4 are provided for generating a plurality of Raman excitation light wavelengths I-1 to I-4.
In view of the total gain characteristic in the above-mentioned case, superimposed gain of the plurality of wavelengths becomes larger as the wavelength becomes shorter. As a result, when assuming each output power of laser diodes LD1 to LD4 for producing excitation light is identical, superimposed total gain becomes different depending on the combinations of different wavelengths. This produces a tilt as shown xe2x80x98Axe2x80x99 in FIG. 3. To cope with this problem in actual implementation, each output power of laser diodes LD1 to LD4 for producing excitation light is individually monitored to adjust so that the tilt may not be produced between each main signal, thus producing substantially flat gain.
Another problem is that, in case of multiple wavelengths, signal-to-noise (SN) ratio disperses depending on channels CH, resulting in deterioration of transmission quality as a whole.
In order to compensate this, a weighted power is applied to each signal in a transmission side to improve SN ratio. This control is called as the pre-emphasis control.
Also, there may be a case that the SN ratio becomes deteriorated unintentionally depending on the shape of the tilt in the above-mentioned Raman amplification. To cope with this in the conventional system, there may be introduced a method shown in FIG. 4 to compensate the tilt produced in total gain.
In FIG. 4, an example of the compensation method is now assumed, which is applicable in a conventional system when the tilt is produced in total gain. Especially in FIG. 4, there are shown configuration examples of a unit of optical amplifier 20 (hereafter referred to as optical amplification unit 20) of FIG. 1 and a unit of Raman excitation light source 22 (hereafter referred to as Raman excitation light source unit 22).
In FIG. 4, in order to conduct power control of each laser diode LD1 to LD4 for producing excitation light, there are provided couplers (CPL) 201, 221 to 223 and photodetectors (PD) 224 to 227 for monitoring the light power of each wavelength, in Raman excitation light source unit 22 and optical amplification unit 20. In optical amplification unit 20, excitation light is multiplexed into optical transmission line 3. Also main signal light is received and extracted.
Excitation light emitted by each laser diode LD1 to LD4 with power control is multiplexed in coupler (CPL) 221. Before the excitation light is output to optical transmission line 3 through optical amplification unit 20, total power is monitored by a photodetector (PD) 220 provided in Raman excitation light source unit 22 for controlling the output.
Here, according to the assumed configuration shown in FIG. 4, the following problems may be pointed out.
(1) a large number of couplers CPL and photodetectors PD are required for monitoring total and individual power of excitation light, which brings about increase of equipment cost.
(2) In Raman amplification method requiring high output power for transmission, an important point is how a loss to be produced in the outputs from laser diodes LD1 to LD4 for producing excitation light before reaching optical transmission line 3 can be eliminated. However the assumed configuration shown in FIG. 4, the loss of the outputs from laser diodes LD1 to LD4 produced before reaching optical transmission line 3 comes to 3 to 4 dB.
If a coupler CPL of which branching ratio is approximately 100:1 (=20 dB) is used, a power of 0 to 10 dBm (=1 to 10 mW) is consumed in a photodetector (PD) for monitoring, which produces a great disadvantage.
(3) A coupler/PD portion having the ratio of 100:1 (=20 dB) produces a dispersion of xc2x11 dB, in the worst case, against the monitoring value a teach PD. This is caused by various dispersions in the branching ratio of the coupler, loss of the coupler itself, a splice connection loss in manufacturing, Quantum Efficiency of PD, etc. Especially, when monitoring the Raman excitation light multiplexed by a coupler (i.e. monitoring by photodetector 220 or 202 in FIG. 4), there exists a dispersion of power in each wavelength input to photodetector 220 or 202 for monitoring.
Therefore, it is difficult to monitor total power accurately. In general, a variation of 1 dB in the excitation light power corresponds to a variation of 2 to 3 dB in the gain in the case of Raman excitation (when a backward excitation shown in FIG. 1 is applied), although the above figure depends on excitation light power or transmission distance. As a result, in the method shown in FIG. 4, it is difficult to control total power value using feedback control.
(4) Each laser diode LD outputs the power of approximately 15 to 25 dBm in maximum per diode, and the power of multiplexed wave generated by a plurality of laser diodes LD reaches as much as 20 to 30 dBm. In photodetectors 220, 202 for monitoring this, a wide dynamic range from output 0xe2x86x92safety light levelxe2x86x92maximum power is required.
Assuming that 4 V corresponds to a full range and is 30 dBm (=1,000 mW), a monitored voltage of the safety light level (in the order of 3 to 5 dBm=2 to 3.2 mW) is as small produce a large readout error and therefore it is difficult to control safety light level with required accuracy.
(5) In the assumed configuration shown in FIG. 4, a large number of components are required from the output portion of laser diodes LD1 to LD4 to the transmission portion of excitation light to optical transmission line 3. These components include an optical system consisting of lens for laser diode module, etc., couplers, photodiodes PD for monitoring, splices, connector joints, etc. When a failure occurs in any of these components, the normal control becomes lost. In such a system, therefore, expecting high reliability may become difficult.
(6) A dynamic range in the input signal of optical amplification unit 20 is several decibels. It is however probable, when the main signal level exceeds the upper limit or the lower limit of the dynamic range, transmission quality may be deteriorated (in view of the noise figure characteristic NF, etc.)
Accordingly, it is an object of the present invention to provide a method for Raman excitation control and optical transmission system using the method to solve the aforementioned problems (1) to (6).
According to the present invention, a Raman excitation control method and optical transmission system using this control method aims at Raman excitation control for use in optical signal amplification by Raman excitation light fed into a transmission line.
The method includes the steps of detecting with a back power monitor backward Raman excitation lights produced by a laser diode circuit; and controlling light emission power of the laser diode circuit according to level of the detected backward lights the back power monitor.
As a preferred embodiment of the present invention, the laser diode circuit includes a plurality of laser diodes, which produce Raman excitation lights having respectively different wavelengths and in the step of detecting, the back power monitor circuit detects the back power lights produced by the plurality of laser diodes and in the step of controlling, light emission power of each of the laser diodes is controlled according to each of the back power lights detected.
As another preferred embodiment of the present invention, a Raman excitation control method for use in optical signal amplification by Raman excitation light fed into a transmission line includes the steps of: detecting with a power monitor a Raman excitation light produced by a laser diode; comparing the power value detected by the power monitor with an initial set data for the laser diode producing the Raman excitation light being stored in a memory; and controlling light emission power of the laser diode according to the result of the comparison.
As still another preferred embodiment of the present invention, a Raman excitation control method for use in optical signal amplification by Raman excitation light fed into a transmission line includes the steps of: detecting with a power monitor Raman excitation lights having different wavelengths produced by a plurality of laser diodes; comparing respective power values detected by the power monitor with initial set data stored in a memory corresponding to each of the plurality of laser diodes producing the Raman excitation lights; and performing a feedback control of light emission power of each plurality of laser diodes according to the result of the comparison.
Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings.